1. Field of the Invention
The present invention relates to a semiconductor laser diode, and more particularly to a semiconductor laser diode in which higher-order mode absorption layers of a super-lattice structure are formed, thereby being capable of achieving an improvement in kink level and an improvement in catastrophic optical damage (COD) level.
2. Description of the Related Art
Generally, semiconductor laser devices are used for light sources of optical pickup devices in optical disc systems such as CD-RW and DVD-RW. Also, they are widely applied to diverse technical fields such as information and image processing, measuring, communication, and medical treatment. Such semiconductor laser diodes used in diverse technical fields are required to exhibit high power, a high COD level, and a high kink level.
FIG. 1 illustrates an example of a conventional high power semiconductor laser diode. As shown in FIG. 1, the high power semiconductor laser diode includes a first-conductivity type semiconductor substrate 12 formed with a first electrode 11 at a lower surface thereof. The high power semiconductor laser diode also includes a first-conductivity type clad layer 13, an active layer 14 having a multi-quantum well structure, and a second-conductivity type clad layer 15 formed with a ridge 15a, which are sequentially formed over an upper surface of the first-conductivity type semiconductor substrate 12. A current confining layer 16 is formed on the second-conductivity type clad layer 15 around the ridge 15a. A contact layer 17 is also formed to cover the upper surfaces of the current confining layer 16 and ridge 15a. A second electrode 18 is formed over the contact layer 17.
In the conventional semiconductor laser diode having the above mentioned structure, its ridge 15a is narrow while having a sharp slope in order to achieve an increase in laser power. In order to form the ridge 15a having such a sharp slope, a dry etching process using plasma is carried out. Where such a narrow ridge having a sharp slope is used, it is possible to achieve an improvement in kink level and an increase in laser power because the laser generating region x in the active layer 14 is narrowed. However, such a narrow laser generating region causes an increase in the optical density per area, thereby resulting in an increase in COD. As a result, a decrease in optical power occurs. In severe cases, oscillation of light may be ceased. This is because the light oscillated from the active layer may be absorbed into the light emitting facet of the semiconductor laser diode, thereby increasing the temperature of the light emitting facet, or fusing the light emitting facet in severe cases, and thus, breaking the device.
Furthermore, where the dry etching process using plasma, damage caused by the plasma remains at the etched region. As a result, there is an adverse affect on the characteristics of the semiconductor laser diode and a degradation in the reliability of the semiconductor laser diode.
FIG. 2 illustrates another example of a conventional high power laser diode. As shown in FIG. 2, the high power semiconductor laser diode includes a first-conductivity type semiconductor substrate 22 formed with a first electrode 21 at a lower surface thereof. The high power semiconductor laser diode also includes a first-conductivity type clad layer 23, an active layer 24 having a multi-quantum well structure, and a second-conductivity type clad layer 25 formed with a ridge 25a, which are sequentially formed over an upper surface of the first-conductivity type semiconductor substrate 22. A light confining layer 26 is formed over the second-conductivity type clad layer 25. The light confining layer 26 has a ridge 26a having a width larger than that of the ridge 25a formed at the second-conductivity type clad layer 25 while having a refractive index lower than that of the second-conductivity type clad layer 25. A current confining layer 27 made of a first-conductivity type semiconductor material is formed on the light confining layer 26 around the ridge 26a. 
In this semiconductor laser diode, formation of a desired ridge structure is achieved by forming the ridge 25a made of a material having a high refractive index at the second-conductivity type clad layer 25 through a wet etching process, forming the light confining layer 26 having a refractive index lower than that of the second-conductivity type clad layer 25 through a secondary growth process without using any oxide film, and subsequently forming the current confining layer 27 made of a first-conductivity type semiconductor material through a third growth process. In the semiconductor laser diode having such a structure, the ridge 25a formed at the second-conductivity type clad layer 25 forms a light confining region y, whereas the ridge 26a formed at the light confining layer 26 forms a current confining region z.
Thus, the above mentioned conventional semiconductor laser diode increases the width of the ridge structure by separating regions for carrier confinement and optical confinement from each other, thereby achieving an improvement in COD level. In this semiconductor laser diode in which its optical density per area is reduced in accordance with an increase in ridge width, thereby achieving an improvement in COD level, however, there is a problem in that when the optical power of oscillated lasers increases, higher-order modes may be generated, thereby causing a degradation in kink level. Now, generation of a kink will be described in detail with reference to FIGS. 3a and 3b. 
FIG. 3a is a graph depicting a variation in laser power in a lateral (width) direction of a semiconductor laser diode. When the semiconductor laser diode begins to operate, and subsequently performs a normal operation, the laser power thereof is exhibited in the form of a Gauss curve. That is, the laser power has a peak at the width center of the active layer, that is, a portion of the active layer corresponding to the center of the ridge width, as shown in FIG. 3a. This laser power is referred to as a “fundamental mode laser power”. However, when the laser power increases, respective numbers of electrons and holes present at the width center of the active layer are decreased. When such a reduction in the numbers of electrons and holes reaches a certain level, two laser power peaks are exhibited at portions of the active layer spaced apart from the width center of the active layer by a certain distance at opposite sides of the width center of the active layer, respectively (regions A and B). The laser power with such peaks is referred to as a first-order mode laser power”. As the laser power further increases, the number of peaks increases. Such modes other than the fundamental mode are referred to as “higher-order modes”. In such higher-order modes, the output power of the semiconductor laser diode may be degraded. Such a problem is called a “kink”.
FIG. 3b is a graph depicting the relation between the current injected into the semiconductor laser diode and the optical power of the semiconductor laser diode. Referring to FIG. 3b, it can be seen that although the optical power of the oscillated laser increases linearly to a certain level Po in accordance with an increase in the amount of the injected current, such a linear increase is no longer obtained after the optical power reaches the level Po. This is because a kink occurs due to oscillation in a higher-order mode. In FIG. 3b, the bent graph portion corresponds to a point where a kink occurs.
Thus, a kink occurs when oscillation in a higher-order mode occurs. Due to such a kink, there is a problem of a degradation in the laser characteristics associated with irradiation position of a laser, laser power, etc. Accordingly, it is important to raise the level at which a kink occurs (that is, the kink level), so as to achieve a stable oscillation of a high-power laser.
Therefore, in the technical field, it has been required to provide a new semiconductor laser diode which can prevent generation of COD while suppressing oscillation in a higher-order mode even when a high-power laser is generated, thereby raising the kink level, so that it can stably oscillate a high-power laser.